Experiments conducted in a piston cylinder apparatus at 10.5 kbar, 1000 °C, reveal that the exchange of Fe and Mg between olivine and clinopyroxene is nearly ideal. Fifty-three experimental half-reversals tightly constrain the curve ln Kd vs. Fe/(Fe + Mg). If WFaF0 = 4500 J/atom, the weighted least-squares regression of the experimental results yields WHdHi = 3978 ± 423 J/atom and ΔGrxn0=7208±174 J/mol for the exchange reaction. A Monte Carlo regression yields similar results. Fe-Mg mixing in olivine, in the M1 site of clinopyroxene, in garnet, and in orthopyroxene can be characterized by small symmetrical Margules parameters: 4500, 3978, 1590, and 0 J/atom, respectively.

Attempts to perform similar experiments on Fe-Mg exchange between garnet and clinopyroxene were unsuccessful. However, the olivine-clinopyroxene results can be combined with the results of other studies on Fe-Mg exchange between garnet and olivine to calculate a garnet-clinopyroxene thermometer. The calculated thermometer is highly uncertain but yields temperatures 30–190 °C higher than those suggested by some recently published experimental results. One of the recent studies is likely in error; more experimental studies are needed to resolve the discrepancres.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.